Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser
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Two‑dimensional laser‑induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing Manu Mannazhi1 · Per‑Erik Bengtsson1 Received: 22 July 2020 / Accepted: 26 October 2020 © The Author(s) 2020
Abstract Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.
1 Introduction Laser-induced incandescence (LII) has been one of the most prominent laser diagnostic techniques for performing quantitative two-dimensional (2D) soot measurements in the past few decades [1, 2]. In this technique, soot particles are heated up to high temperatures (~ 4000 K) using high-energy laser pulses. These laser - heated soot particles emit enhanced Planck radiation with respect to the flame soot, and this enhanced radiation is called the LII signal. The LII signal has been found to be proportional to soot volume fraction ( fv ), to a first approximation [3], which can be calibrated to absolute values using, for example, extinction measurements [4]. Although LII appears to be a simple technique, there are several challenges in performing accurate quantitative 2D LII measurements. As soot is a strong absorber, the attenuation of the laser beam along the beam * Manu Mannazhi [email protected] Per‑Erik Bengtsson per‑[email protected] 1
Division of Combustion Physics, Department of Physics, Box 118, Lund University, SE‑22100 Lund, Sweden
propagation direction as well as LII signal trapping from the measurement region to the detector are potential problems [5, 6]. Another problem often appears in pressurised systems where optical windows are used, where soot contamination commonly appears on the detection window [7]. Furthermore, as combustion environments have strong temperature and density gradients, beam steering may also lower the accuracy in soot volume fraction measurements in p
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